I got a question about comets,
so I spent an hour on them. Of course 1997 was the year of Hale-Bopp,
and some of the children remembered seeing that one. Almost no one
remembered Hayakutake from 1996, which surprised me a bit. I brought in
a cooler with dirty snow to show them how loose and light comets are.
We talked about where they come from (the Oort cloud), and how big this
cloud is relative to the biggest planetary orbit, and how the Oort cloud
got formed, and about the fact that comets are representative of the
stuff that the solar system was built from, and that NASA was going to
fetch a piece of a comet soon. This is the time to stress the
difference between comets and asteroids, which fall on the earth, and can
be as dense as the crowbar I brought in, just to show the difference with
(the bulk of) cometary material. (more text to come...)

We spent an hour on the subject of sound.
This time I violated my no-tech rule, because I brought in
my old scope and a microphone. This was a great hit, as the microphone
was passed around, and the kids made high notes, low notes, whistled etc
and saw little waves, big waves and all that on the screen.
I had also brought in some 78 rpm
records, and magnifying glasses. The grooves on 78's are coarse enough
so that you see the wiggles in the grooves. Then I played one of those
records, using a pencil, a regular sheet of paper and a straightpin.

I did an hour on sound again the next year, with some different demos, and
some the same.
Take a look here.

Eclipses, and phases of the moon

Good timing in both cases, because in '98 a total solar eclipse
was going to occur on the
October 26th, unfortunately only partial here in New Mexico (you'd have to go to Panama,
Columbia, Venezuela, or some Carribean islands to see the total eclipse).
In October 2004 a total lunar eclipse
occurred that same week.
Here is
a great introduction
to eclipse causes, effects and patterns.
How to show eclipses in the classroom - especially when the room cannot be
darkened very well? I got a 120W spotlight which I put into a droplight
fitting, which I clamped onto my tripod. This made an adequately bright
sun. I brought in a globe, and a little lego guy (with sticky feet) to
stand on it, and a variety of little and medium balls I had around the
house. The marble-sized beads are good to show the umbra and the
penumbra, and I had some white styrofoam balls (erstwhile christmas tree
decorations), which are great to show the phases of the moon with. A
coathanger to stick the balls onto comes in handy when you need to make
them fly around the globe. This is pretty much all you need to
explain the phases of the moon, and incidentally also that the planets
interior to the earth (Mercury and Venus) show phases just like the moon,
and why the outer planets don't. You can show that while solar eclipses
can be seen only if you are in the right place, lunar eclipses can be
seen from any place on earth (well, only half). You can show how total,
partial and annular eclipses work. While I had the stuff out, it was
easy to show why we can't see the backside of the moon. After all this,
you can see if they can figure out why solar eclipses only occur at the
new moon, and lunar eclipses only at the time of the full moon. If they
got this right, they really undestood the whole story. Here is a
bunch of links with pretty pictures, and lots more:

I did this one again on 5 Jan 99, and updated the '99 links. Updated October 2004.

February 98: Where do all the animals come from?
Time for a break from all the space stuff. There had been some
other vague questions about animals and plants, so here was an
opportunity to talk about evolution, one of the great pillars of science
that the kids should be familiar with. I decided to make up a game
that could be played in class that would show evolutionary pressure,
drift, divergence and all that good stuff. Here is how it went:

First I set the scene: We have a small green valley, with brush, trees and
grass, all drawn on the board, and we are going to play at being these
medium-sized herbivores. They live for 4 years, then they die.

To make it easier for the children to keep track of their 'age',
I folded colored sheets in four, and marked quarters with large numbers
1-4, so that they could fold the right number out. Each 'live' animal
gets to hold one.

In the first phase of
the game, we're basically going to populate the imaginary valley
up to the limit of the food supply, and follow the population for a while.
Each time we turn over a year, all live animals turn a year older,
and get to multiply - that is, pick another kid to join the herd, who
starts off at age 1. Those who turn 5, die and leave the herd. The valley
can support 14 (that's how many age-cards I made). Of course it takes
only 4 generations to saturate the food supply, after which the
new-member rate equals the death rate. This gets pretty boring after a
few rounds, but it shows that a population will fill an unoccupied
environmental niche, and then becomes resource-limited.
In the next phase, we're going to throw in some random
variation. In addition to the age-cards, I had made up 14 simple tape
measures

Colored 8.5x11 sheets from the recycling, cut into 4 lengthwise strips.
Tape 7 different colored strips end-to-end, fold them up and hold them
together with a clothes pin. Unfold as many strips as you need 'feet',
and put the clothespin back on the rest.

The 'live' animals each get a tape measure so
they can keep track of how tall they are. They all start out at 3', so
they can eat the shrubbery, but cannot reach the leaves on the trees. Now,
every time someone gets to pick an offspring, they get to roll a die;
I had some 8-sided dice, and made the following assignments:

throw 1-2: offspring is 1' shorter than the parent

throw 3-6: offspring is the same length as the parent

throw 7-8: offspring is 1' taller than the parent

After a few rounds/years, the population is still mostly 3', but there
are now a few tall and short members.

Then evolutionary pressure strikes: the climate in our
little valley changes, and it turns into a a plain with a little grass
and trees, but no more brush. There is not enough food for the medium-sized
brush eaters, and they die off. The taller animals get to munch on the
trees, and a few of the smallest animals get to survive on the grass.
After a few rounds/years/generations it becomes clear that we now have
two different types of animals where we started with one type. Just
throwing the dice and some environmental pressure led to emergence of new
species. That's just about all we got done in one hour.

The next
week, I reviewed the game, and put the proper names to various
things we had done. While we were playing the game the week before,
I had had no time to introduce the terminology ('evolution', 'survival of
the fittest' etc.), or any of the names (Darwin, Wallace). By coincidence,
I had just received the Feb 98 issue of Scientific American, in which
there was an article about the emergence of antibiotic resistance - a
prime example of evolution in real-time action in our world, and I used
that to stress that evolution is not just a thing of the past. Also, that
under current circumstances, that the human race can no longer evolve.

Of course, while we were on the subject, I had to mention DNA,
stepping down in scale from my fingertip to cells, then nuclei and
finally DNA. I had made up some new covers for a set of paperback books
of various thicknesses, a fat one titled 'How to make a Person', an
equally thick volume 'How to make a Monkey', a thinner one 'How to make
a Frog, and a single sheet titled 'How to make Bacteria'. These codebooks
are supposed to be written with only the letters C,G,T and A. Another
important point about these codebooks is that they contain many almost
identical chapters and pages between them, even between the 'Person' code
and the 'Bacteria' code. (Feb 16 98)

Finally, a collection
of related links:

Charles Darwin. The text of his works: 'The voyage of the Beagle',
'The origin of the species', and 'the Descent of Man'.